The invention relates generally to wireless communications. More particularly, the invention relates to scheduling of data wirelessly transmitted between a base control station having multiple antennas, and subscriber units.
Wireless communication systems commonly include information carrying modulated carrier signals that are wirelessly transmitted from a transmission source (for example, a base transceiver station) to one or more subscribers (for example, subscriber units) within an area or region.
Spatial multiplexing is a transmission technology that exploits multiple antennae at both the base transceiver station and at the subscriber units to increase the bit rate in a wireless radio link with no additional power or bandwidth consumption. Under certain conditions, spatial multiplexing offers a linear increase in spectrum efficiency with the number of antennae. The substreams occupy the same channel of a multiple access protocol, the same time slot in a time-division multiple access protocol, the same frequency slot in frequency-division multiple access protocol, the same code sequence in code-division multiple access protocol or the same spatial target location in space-division multiple access protocol. The substreams are applied separately to the transmit antennae and transmitted through a radio channel. Due to the presence of various scattering objects in the environment, each signal experiences multipath propagation.
The composite signals resulting from the transmission are finally captured by an array of receiving antennae with random phase and amplitudes. At the subscriber array, a spatial signature of each of the received signals is estimated. Based on the spatial signatures, a signal processing technique is applied to separate the signals, recovering the original substreams.
FIG. 1 shows three transmitter antenna arrays 110, 120, 130. The transmitter antenna arrays 110, 120, 130 transmit data symbols to a subscriber antenna array 140. Each transmitter antenna array includes spatially separate antennae. A subscriber connected to the subscriber antenna array 140 separates the received signals.
FIG. 2 shows modulated carrier signals traveling from a transmitter 210 to a subscriber 220 following many different (multiple) transmission paths.
Multipath can include a composition of a primary signal plus duplicate or echoed images caused by reflections of signals off objects between the transmitter and subscriber. The subscriber may receive the primary signal sent by the transmitter, but also receives secondary signals that are reflected off objects located in the signal path. The reflected signals arrive at the subscriber later than the primary signal. Due to this misalignment, the multipath signals can cause intersymbol interference or distortion of the received signal.
The actual received signal can include a combination of a primary and several reflected signals. Because the distance traveled by the original signal is shorter than the reflected signals, the signals are received at different times. The time difference between the first received and the last received signal is called the delay spread and can be as great as several microseconds.
The multiple paths traveled by the modulated carrier signal typically results in fading of the modulated carrier signal. Fading causes the modulated carrier signal to attenuate in amplitude when multiple paths subtractively combine.
Antenna diversity is a technique used in multiple antenna-based communication system to reduce the effects of multi-path fading. Antenna diversity can be obtained by providing a transmitter and/or a subscriber with two or more antennae. These multiple antennae imply multiple channels that suffer from fading in a statistically independent manner. Therefore, when one channel is fading due to the destructive effects of multipath interference, another of the channels is unlikely to be suffering from fading simultaneously. By virtue of the redundancy provided by these independent channels, a subscriber can often reduce the detrimental effects of fading.
Wireless systems generally require scheduling of information transmitted between base transceiver stations and subscriber units. The bandwidth of the available transmission frequencies is limited. Therefore, the transmission between multiple transceiver stations and subscriber units generally requires time, frequency, or some other type of multiplexing. The larger the number of base station transceivers and subscriber units, the more complex the scheduling. Additionally, the above-described spatial multiplexing and communication diversity add complexity to the scheduling.
An individual transmission link exists between each individual base transceiver station antenna and a subscriber unit in communication with the base transceiver station. The previously described spatial multiplexing and communication diversity require multiple antennas to each have transmission links with a single subscriber unit. Optimally, the base transceiver station can schedule data transmission according to the transmission link quality between each base transceiver station antenna the subscriber unit. That is, the amount of information that can be transmitted between the individual base transceiver station antennas and the subscriber unit is base upon the quality of the transmission links. Ideally, the scheduling of the transmission of information between the base station transceiver and the subscriber units is dependent upon the quality of the individual transmission links.
It is desirable to have an apparatus and method that provides scheduling of transmission of data blocks between base station transceiver antennas and subscribers (subscriber) units. It is desirable that the scheduling be adaptive to the quality of transmission links between the base station transceiver antennas and each of the subscribers (subscriber) units. It is additionally desirable that the apparatus and method allow for spatial multiplexing and communication diversity.
The invention includes an apparatus and a method for scheduling wireless transmission of data blocks between at least one antenna of a base transceiver station and multiple subscriber units. The scheduling can be based on the quality of a transmission link between the base station antennas and the subscriber units, the amount of data requested by the subscriber units, and/or the type of data requested by the subscriber units. The scheduling generally includes assigning frequency blocks and time slots to each of the subscriber units for receiving or transmitting data blocks.
A first embodiment of the invention includes a method for transmitting data streams between a base transceiver station and a plurality of subscribers. The method includes receiving protocol data units from a network, creating sub-protocol data units from the protocol data units, and once per a frame of time, generating a schedule that designates time slots and pre-defined frequency blocks in which each one of the plurality of subscribers is to receive each of the sub-protocol data units from a plurality of base station transceiver antennas.
A second embodiment of the invention is similar to the first embodiment. The second embodiment further includes transmitting the schedule to each of the subscribers, and the plurality of base station transceiver antennas transmitting the sub-protocol data units according to the schedule.
A third embodiment is similar to the second embodiment. The third embodiment includes selecting at least one transmission mode for each subscriber. The transmission mode dictating the type of modulation and/or coding used during transmission of the sub-protocol data units. The transmission mode selection can be dependent upon a quality of transmission link between the base station transceiver and the subscribers, and/or a quality of service requested by the subscribers.
A fourth embodiment is similar to the first embodiment. For the third embodiment, generating a schedule that designates time slots and pre-defined frequency blocks includes receiving service flow requests from the subscribers. The service flow requests indicating demands for data by the subscribers. Generating a schedule can further include receiving an information size request from the subscribers, and/or receiving a block weight for each of the service flow requests, wherein the block weight is dependent upon a priority of each of the service flow request. The block weight determines how many consecutive time slots and frequency blocks are transmitted to each subscriber.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.